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Abstract

Microresonator frequency combs (microcombs) are enabling new applications in frequency synthesis and metrology – from high-speed laser ranging to coherent optical communications. One critical parameter that dictates the performance of the microcomb is the optical quality factor (Q) of the microresonator. Microresonators fabricated in planar structures such as silicon nitride (Si3N4) allow for dispersion engineering and the possibility to monolithically integrate the microcomb with other photonic devices. However, the relatively large refractive index contrast and the tight optical confinement required for dispersion engineering make it challenging to attain Si3N4 microresonators with Qs > 107 using standard subtractive processing methods – i.e. photonic devices are patterned directly on the as-deposited Si3N4 film. In this work, we achieve ultra-smooth Si3N4 microresonators featuring mean intrinsic Qs around 11 million. The cross-section geometry can be precisely engineered in the telecommunications band to achieve either normal or anomalous dispersion, and we demonstrate the generation of mode-locked dark-pulse Kerr combs as well as soliton microcombs. Such high-Qs allow us to generate 100 GHz soliton microcombs, demonstrated here for the first time in Si3N4 microresonators fabricated using a subtractive processing method. These results enhance the possibilities for co-integration of microcombs with high-performance photonic devices, such as narrow-linewidth external-cavity diode lasers, ultra-narrow filters and demultiplexers.

Figures (4)

Fig. 1. (a) Fabrication flow of Si3N4 waveguides starting from deposition of crack-free thin film until overclad of Si3N4 waveguide. (b) Perspective view SEM image of fabricated Si3N4 waveguide. Little roughness can be observed, and no clear interface between two Si3N4 layers can be seen. (c) AFM measurement of the top surface of Si3N4 with thickness ∼ 600 nm. (d) Top view SEM image of fabricated Si3N4 waveguide, the magnified waveguide image is in the inset. (e) SEM image of coupling region milled by FIB. The Si3N4 bus and ring waveguides are painted with purple color. The gap between ring and bus waveguides is ∼ 400 nm, and no air void has been observed.

Fig. 4. Soliton comb generation from microring resonator with FSR 100 GHz with fixed pump laser and on-chip thermal heater. (a) The signal for microheater to generate a soliton microcomb. The corresponding converted power trace is shown in (b). (c) Zoomed in trace in (b) showing the single soliton step. (d) Single soliton comb generated with on-chip power ∼ 100 mW. The microscope image of microheater is in the inset. (e) Corresponding RF spectrum.